1. Field of the Invention
The present invention relates to an acceleration slip control system for a motor vehicle, and more particularly to one of the type in which the hydraulic pressure of each wheel cylinder is reduced after the end of an acceleration slip control.
2. Description of the Prior Art
In general, in the mode of an acceleration slip control for a motor vehicle, the communication between a master cylinder (which is communicated with a brake pedal) and each wheel cylinder is cut off, and a hydraulic pressure source for the acceleration slip control and the wheel cylinder are brought into communication, whereupon the hydraulic pressure of the wheel cylinder is controlled. After the end of the acceleration slip control, the master cylinder and the wheel cylinder are brought into communication again. However, when a high residual pressure exists in the wheel cylinder or a pipe on this occasion, it flows into the master cylinder. It is therefore apprehended that noise will be produced or that the cup of the piston of the master cylinder will be damaged.
In this regard, with the intention of protecting the master cylinder, a control system is disclosed in, for example, the official gazette of Japanese Patent Application Laid-open No. 91196/1996. Herein, after the end of the acceleration slip control, the hydraulic pressure of the wheel cylinder is sufficiently reduced, thereafter, the master cylinder and the wheel cylinder are communicated.
According to the known control system, in the mode of the acceleration slip control, a fluid passage which joins the master cylinder with the wheel cylinder is blockaded by a master cut valve. Under this condition, the discharge side of a pump is connected between the master cut valve and the wheel cylinder so as to transmit the discharge pressure thereof to the wheel cylinder.
At the end of the acceleration slip control, a fluid passage part which joins the wheel cylinder with the reservoir of the master cylinder (master reservoir) is opened by a releasing device for a predetermined time period, thereby to sufficiently reduce the hydraulic pressure of the wheel cylinder. Thereafter, the master cut valve is opened.
With the control system disclosed in the above official gazette, the predetermined time period (release time period of the releasing means) for reducing the hydraulic pressure of the wheel cylinder is set equally or indiscriminately.
However, when the predetermined time period for the pressure reduction is set to be somewhat long, the master cylinder and the wheel cylinder are separated unnecessarily long in some drive environments. In consequence, if the brakes are applied in the course of the pressure reduction, the effective braking of driving wheels will delay in correspondence with the operating procedures in which the hydraulic pressure of the wheel cylinder is sufficiently reduced, and the released or OFF state is changed-over into an applied or ON state so that the master cylinder and the wheel cylinder fall into the communicating state. Regarding the delay, therefore, said predetermined time period should preferably be as short as possible.
However, when the predetermined time period is set to be somewhat short, the master cylinder and the wheel cylinder are brought into communication before the sufficient reduction of the residual pressure, in an exemplary situation where the temperature of brake fluid is low or where a very high hydraulic pressure has been exerted on the wheel cylinder. It is therefore impossible to attain the effects of the control originally intended.